ProjectSummary Embryoimplantationfailureisasignificantcausalfactorofinfertilityforwomenworldwide.Althoughadvances inourunderstandingofoocyteandembryodevelopmenthaveimprovedpregnancysuccessrates,theserates remain unacceptably low due in part to an endometrium that is nonreceptive to the embryo. For successful implantation, endometrial receptivity and subsequent decidualization requires coordinated progesterone (P4) signaling in a cell-type specific manner. While the signature cellular events that underpin P4-driven uterine receptivity and decidualization are known, our knowledge of the pivotal mediators of P4 action in these processes is incomplete. This knowledge-deficiency is significant as nothing short of identifying the key early signalsthatunderpinP4-drivenuterinereceptivitywilladdressthecurrentclinicallimitationsindiagnosingand treating a non-receptive uterus at the molecular level. To address this deficiency, we recently demonstrated that the promyelocytic leukemia zinc finger (PLZF) transcription factor is a direct target of the progesterone receptor (PGR) and is indispensable for P4-dependent decidualization ofcultured human endometrial stromal cells (hESCs). As further translational support for a P4 mediator role for PLZF in the human endometrium, PLZF expression levels in human endometrial biopsies are significantly induced during the P4-dominant secretoryphaseofthehumannon-conceptionmenstrualcycle.Intheearlypregnantmouse,Plzfisinducedin theepithelialandstromalcompartmentsofthereceptiveuterusandisstronglyexpressedindecidualcellswith pregnancy progression. These findings support our hypothesis that PLZF (and its downstream transcriptional program)actsasapivotalmediatorofP4-dependentuterinereceptivityanddecidualizationanddoessoinan endometrial cell-type specific manner. This hypothesis will be tested by three specific aims. Using a recently generatedmousemodelcarryingaPlzfconditionalallele,SpecificAim1willestablishtheinvivoimportance of Plzf (and its transcriptional programs) in P4-dependent endometrial receptivity and decidualization. DissectingtheindividualcontributionsofepithelialandstromalPlzfsignalinginthemurineendometriumduring the periimplantation period will be a major focus of Specific Aim 2. We recently demonstrated that direct transcriptional repression of the early growth response 1(EGR1) transcription factor by PLZF is required for hESCdecidualization;?blockingthisregulationimpairshESCdecidualization.Priortodecidualization,however, EGR1inpre-decidualhESCsisrequiredforthesecellstodecidualize,suggestingthatEGR1?primes?thepre- decidualhESCfordecidualization.SpecificAim3willaddressthisproposalbymolecularlycharacterizingthe P4-PGR-PLZF-EGR1 regulatory axis that is required for in vitro and in vivo decidualization. These aims will use state-of-the-art engineered mice to study the role of Plzf in endometrial receptivity and decidualization as well as high throughput genome-scale approaches to identify novel endometrial targets directly regulated by Plzfandthetranscriptionalinteractomethroughwhichthisregulationoccurs.